Method of manufacturing piezoelectric component

ABSTRACT

A method of manufacturing a piezoelectric component includes forming an unhardened first elastic material partially on at least a pair of end portions of a piezoelectric element, the pair of end portions including an edge portion of the piezoelectric element, hardening the first elastic material, forming an unhardened second elastic material on the entire circumference of the piezoelectric element and the first elastic material, hardening the second elastic material, forming an unhardened outer-cladding resin on the entire circumference of the second elastic material covering the piezoelectric element and the first elastic material, and hardening the outer-cladding resin.

This Application is a Divisional Application of U.S. patent applicationSer. No. 09/412,204, filed on Oct. 5, 1999, now U.S. Pat. No. 6,344,706.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric component and a methodof manufacturing the piezoelectric component. In the piezoelectriccomponent, a periphery of a piezoelectric element is covered by anelastic material, and the circumference of the elastic material and thepiezoelectric element is covered by an outer-cladding resin.

2. Description of the Related Art

Japanese Unexamined Patent Publication No. 1-228310 describes apiezoelectric component in which a periphery of a piezoelectric elementhaving end electrodes connected to lead terminals is covered by anelastic material such as a silicone rubber, and the circumferencethereof is sealed by an outer-cladding resin.

In order to manufacture such a piezoelectric component, first, a leadterminal is attached to a piezoelectric element. Then, a silicone rubberlayer is provided on the piezoelectric element by the dipping thepiezoelectric element in a liquid-state silicone rubber and hardeningthe rubber layer by applying heat. Further, an epoxy resin layer isprovided on the exterior of the silicone rubber via a dipping method.Then, the epoxy resin is hardened by heat to obtain an outer-claddingresin.

The epoxy resin used for forming the outer-cladding resin layercontracts when it is hardened by heat, and thereby a stress caused bythe outer-cladding resin which contracts and compresses the interior ofthe piezoelectric component is generated. If the amount of stress whichcontracts and compresses the piezoelectric element in the piezoelectriccomponent changes, the oscillation characteristics thereof change. Thesilicone rubber is used because it is able to prevent the change of theoscillation characteristics of the piezoelectric element, and has adamping effect which suppresses the waveform distortion of thepiezoelectric element.

To reduce the stress generated in the piezoelectric element by theouter-cladding resin, preferably, the difference between the edgeportion of the piezoelectric element and the outer-cladding resin i.e.,the minimum thickness of the silicone rubber, is larger than thecontraction amount of the outer-cladding resin when it is hardened, andfurther, is larger than the difference between the thermal expansions ofthe outer-cladding resin and of the piezoelectric element.

However, the dipping method for forming the liquid-state silicone rubberwas not satisfactory for obtaining a required thickness of the siliconerubber at the end portions of the piezoelectric element, because of thesurface tension of the silicone rubber. Therefore, the temperaturecharacteristics of the piezoelectric components are largely varied bythe stress generated in the piezoelectric element by the outer-claddingresin.

SUMMARY OF THE INVENTION

To overcome the above described problems, preferred embodiments of thepresent invention provide a piezoelectric component and a method ofmanufacturing the piezoelectric component, which greatly reduces thestress generated in the piezoelectric element by the outer-claddingresin while the function of damping the piezoelectric element ismaintained.

One preferred embodiment of the present invention provides apiezoelectric component, including a substantially rectangular shapedpiezoelectric element, a first elastic material covering at least a pairof end portions of the piezoelectric element, the pair of end portionsincluding edge parts of the piezoelectric element, a second elasticmaterial covering the entire piezoelectric element and the first elasticmaterial, and an outer-cladding resin covering the whole circumferenceof the piezoelectric element covered with the second elastic material.

In the above described piezoelectric component, the end portions of thepiezoelectric element, where it is most difficult to obtain a sufficientthickness of an elastic material, is covered by the first elasticmaterial. Further, the periphery of the first elastic material iscovered by the second elastic material. That is, two elastic materialsare provided at the edge parts of the piezoelectric element, where thepiezoelectric element is most easily affected by the contraction stressimparted by the outer-cladding resin. Therefore, even when thecontraction stress imparted by the outer-cladding resin varies accordingto temperature changes, a change in oscillation characteristics causedby the change of temperature is prevented because the first and secondelastic materials having sufficient thickness absorb any changes ofstress. Further, the damping effect is maintained by the second elasticmaterial to thereby prevent a waveform distortion or other harmfuleffects from being generated.

The modulus of elasticity and the hardness of the first and secondelastic materials are determined in accordance with respective requiredcharacteristics. In the case of the second elastic material. Forexample, it is preferable to set the hardness equal to or less than 28in “Shore Hardness A.”

Another preferred embodiment of the present invention provides a methodof manufacturing a piezoelectric component, including the steps offorming an unhardened first elastic material partially on at least apair of end portions of a piezoelectric element, the pair of endportions including edge parts of the piezoelectric element, thenhardening the first elastic material, forming an unhardened secondelastic material on the entire circumference of the piezoelectricelement and the first elastic material, then hardening the secondelastic material and forming an unhardened outer-cladding resin on theentire circumference of the second elastic material covering thepiezoelectric element and the first elastic material, then hardening theouter-cladding resin.

By the above described method, the piezoelectric component according topreferred embodiments of the present invention can be manufacturedeasily.

For forming the unhardened first and second elastic materials, not onlya dipping method but also using a soldering iron, dispenser, or othersuitable method can be utilized. Similarly, a dipping method or othermethods can be utilized for forming the unhardened outer-cladding resin.

Although the first elastic material and the second elastic material maybe the same, it is preferable to make the thixotropic index (call it athixo index hereafter) of the first elastic material larger than thethixo index of the second elastic material. The thixotropic property isone of the characteristics of fluid, and is defined as a property inwhich a viscosity varies nonlinearly depending on a shearing stress. Itis easy to achieve sufficient thickness of the elastic material whenforming the elastic material on the piezoelectric element if the thixoindex of the elastic material is large. Therefore, if the thixo index ofthe first unhardened elastic material is made larger than the thixoindex of the second unhardened elastic material, sufficient filmthickness at the both edge parts of the piezoelectric element can beobtained. On the other hand, there is no problem even when the thixoindex of the second elastic material is small, because it is easy tocoat the center portion of the piezoelectric element using the secondelastic material, and this construction is hardly affected by heat.

Preferably, the thixo index of the first and second elastic material ispreferably larger than about 1.7. If the thixo index is equal to or lessthan about 1.7, it is hard to achieve a sufficient thickness of theelastic material at the end portions of the piezoelectric elementbecause of a surface tension of the elastic material.

Other features and advantages of the present invention will becomeapparent from the following description of the invention which refers tothe accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a longitudinal sectional view of one preferred embodiment of apiezoelectric component according to the present invention.

FIG. 2 is a sectional view of FIG. 1 taken along the line II—II in FIG.1.

FIG. 3 is a front view of the piezoelectric element shown in FIG. 1 withwhich lead terminals are fixed.

FIG. 4 is a back view of the piezoelectric element shown in FIG. 1 withwhich lead terminals are fixed.

FIG. 5 is a front view of the piezoelectric element with the leadterminals shown in FIG. 3, wherein a first elastic material is furtherprovided at the edge parts of the piezoelectric element.

FIG. 6 is a front view of the piezoelectric element with the leadterminals and the first elastic material shown in FIG. 5, wherein asecond elastic material is further provided on the entire surface of thepiezoelectric element and the first elastic material.

FIG. 7 is a graph showing the relationship between a thixotropic indexand a thickness of the elastic material at the edge portion of thepiezoelectric element.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 to 6 shows a three terminal type piezoelectric trap componentwhich is one preferred embodiment of a piezoelectric component accordingto the present invention. This piezoelectric trap component preferablyincludes a substantially rectangular shaped piezoelectric-ceramicsubstrate 1 which has a pair of substantially rectangular majorsurfaces. The substrate 1 is polarized along the longitudinal directionof the major surfaces thereof. As shown in FIG. 3 and FIG. 4, thedivided electrodes 2 and 3 are disposed on both end portions of a firstof the major surfaces of the piezoelectric-ceramic substrate 1. Theground electrode 4 is disposed at the approximate center portion of asecond major surface of the piezoelectric-ceramic substrate 1. Thedivided electrodes 2 and 3 and the ground electrode 4 are partiallyoverlapping each other with the piezoelectric-ceramic substrate 1disposed therebetween. A thickness shear vibration is generated at theopposite portion. The piezoelectric element of a preferred embodiment ofthe present invention is preferably constituted by thepiezoelectric-ceramic substrate 1 provided with the above describedelectrodes 2-4. Lead terminals 5 and 6 are respectively fixed andconnected to the divided electrodes 2 and 3 via soldering or othersuitable joining process or member. A lead terminal 7 is fixed andconnected to the ground electrode 4. The lead terminals 5-7 arepreferably defined by plate-shaped or wire-shaped metallic material.

Edge portions of the piezoelectric-ceramic substrate 1 are respectivelycovered by silicone rubber layers 10 and 11 which define the firstelastic material 1. The thixo indexes of the silicone rubber 10 and 11in an unhardened state are preferably larger than about 1.7 to achievesufficient thickness of the first elastic material at the edge portionsof the piezoelectric-ceramic substrate 1.

The entire periphery of the piezoelectric-ceramic substrate 1 with thesilicone rubber layers 10 and 11 is preferably covered with a siliconerubber 12 which defines a second elastic material.

Since this silicone rubber 12 covers the vibrating portion of thepiezoelectric element, it is necessary to select a material property inconsideration of the piezoelectric property in addition to thethixotropic property in an unhardened state. In this preferredembodiment, the thixo index of the second silicone rubber 12 ispreferably larger than about 1.7, although it is preferably smaller thanthe thixo index of the silicone rubber layers 10 and 11. Moreover, inthe case of a piezoelectric trap component, in order to prevent thedeterioration of the piezoelectric property, it is preferable to makethe hardness of the second silicone rubber 12 equal to or less thanabout 28 in “Shore Hardness A”.

It is noted that the thixo index used here means a ratio of a viscosityat about 6 rpm and a viscosity at about 60 rpm i.e.,the thixo index (Ti)=a viscosity at about 6 rpm/a viscosity at about 60rpm.

The entire periphery of the piezoelectric-ceramic substrate 1 with thesilicone rubber 12 is covered via an outer-cladding resin 13. In orderto secure an electric insulation, mounting strength, and other desiredcharacteristics, an epoxy resin is preferably used for defining theouter-cladding resin 13, for example.

A preferred embodiment of a method of manufacturing the above describedpiezoelectric trap is explained below. First, a piezoelectric element 1as shown in FIG. 3 and FIG. 4 is prepared. That is, the lead terminals5-7 are respectively fixed and connected to the electrodes 2, 3, and 4respectively disposed on a pair of major surfaces of thepiezoelectric-ceramic substrate 1. Next, as shown in FIG. 5, the firstsilicone rubber 10 and 11 in a fluid state i.e., an unhardened state, ispartially provided so that the end portions of the piezoelectric element1, especially the edge portions of the piezoelectric element 1 arecovered. More specifically, the silicone rubber 10 and 11 in the fluidstate is provided on the edge portions of the piezoelectric element 1via iron, etc., then the silicone rubber layers 10 and 11 are hardened.

Next, as shown in FIG. 6, a second silicone rubber 12 in a fluid stateis provided on the entire surface of the piezoelectric element 1 andthen the second silicone rubber 12 is hardened. This process isperformed preferably by dipping the piezoelectric element 1 into thesecond silicone rubber 12 in the fluid state i.e., an unhardened state.To facilitate the dipping process, the second silicone rubber 12 may bediluted with xylene, or other suitable material or substance.

Next, the piezoelectric element 1 including the second silicone rubber12 is dipped into an outer-cladding resin 13 in a fluid state i.e., anunhardened state. Then, the outer-cladding resin 13 covering thepiezoelectric element 1 and the second silicone rubber 12 is hardenedvia heating. As a result, the piezoelectric trap shown in FIGS. 1 and 2is obtained.

For obtaining a stable piezoelectric property, preferably, the totalthickness of the silicone rubber layers 10, 11 and 12 at the edgeportion 1 a of the piezoelectric element 1 is larger than the differencebetween the maximum contraction amount of the outer-cladding resin 13and the maximum contraction amount of the piezoelectric element 1. Thepreferable thickness can be easily obtained by forming the siliconerubber layers 10, 11 and 12 as described above.

Table 1 shows the relationship between a thixotropic index Ti of asilicone rubber in a fluid state and a mean thickness of the siliconerubber layer at the edge portion of a piezoelectric element.

As apparent from Table 1, the thixotropic index Ti is preferably largerthan about 1.7 in order to achieve a predetermined thickness. If thethixotropic index Ti is about 1.7 or less, the silicone rubber can notbe disposed at the edge portion of a piezoelectric element.

TABLE 1 Mean thickness of Silicone rubber at the Thixotropic index Tiedge portion of a piezoelectric element 3.1 54.1 2.8 52.9 2.5 50.8 2.342.7 2.1 29.5 1.8 8.0 1.7 0

FIG. 7 is a graph showing the relationship indicated in Table 1.

Table 2 shows the relationship between a thickness of a silicone rubberat the edge portion of a piezoelectric element and a Fo temperaturecharacteristic of a piezoelectric trap component. The Fo temperaturecharacteristic is determined by measuring the Fo temperature of thepiezoelectric trap component within a range of about −20 degreescentigrade and about +85 degrees centigrade and then calculating thechanging ratio based on Fo temperature at about +20 degrees centigrade.

As apparent from the results, the standard value of the Fo temperaturei.e., ±0.5%, is satisfied when the thickness of the silicone rubber atthe edge portion of the piezoelectric element is substantially equal toor larger than about 5 micrometers. If a silicone rubber in a fluidstate having a thixo index Ti that is larger than about 1.7 is used, itis easy to achieve a silicone rubber layer at the edge portion of apiezoelectric element having a thickness which is substantially equal toor larger than about 5 micrometers.

TABLE 2 Thickness Fo changing ratio at each temperature (%) (micrometer)−20° C. 0° C. +20° C. +40° C. +60° C. +85° C. 0 0.28 0.18 0.00 −0.24−0.68 −0.79 5 0.03 0.02 0.00 −0.05 −0.14 −0.16 8 −0.05 −0.02 0.00 −0.04−0.18 −0.20 15 −0.08 −0.03 0.00 −0.03 −0.11 −0.12

The present invention is not limited to the above described preferredembodiments. As a piezoelectric element, the present invention is notlimited to a piezoelectric trap component but may be a piezoelectricfilter such as one disclosed in Unexamined Japanese Patent PublicationNo. 1-228310, a ceramic discriminator, or other suitable apparatus.

The number of terminals may be two or three.

It is possible to use the same silicone rubber for forming the firstelastic material and the second elastic material.

As a method for providing the first elastic material, in addition todropping by an iron or a dispenser, dipping the edge portion of thepiezoelectric element into the first elastic material in a fluid stateis also possible. Further, as a method for providing the second elasticmaterial, other methods can be applicable in addition to the dippingmethod.

Further, a material other than silicone rubber is also applicable forthe elastic materials if such material has required characteristic.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the forgoing and other changes in form anddetails may be made therein without departing from the spirit of theinvention.

1. A method of manufacturing a piezoelectric component, comprising thesteps of: forming an unhardened first elastic material partially on atleast a pair of end portions of a piezoelectric element, the pair of endportions including an edge portion of the piezoelectric element;hardening the first elastic material; forming an unhardened secondelastic material on an entire circumference of the piezoelectric elementand the first elastic material; hardening the second elastic material;and forming an unhardened outer-cladding resin on the entirecircumference of the second elastic material covering the piezoelectricelement and the first elastic material; and hardening the outer-claddingresin; wherein the total thickness of first and second elastic materialsat the edge portions of the piezoelectric element is larger than adifference between the maximum contraction amount of the outer-claddingresin and a maximum contraction amount of the piezoelectric element. 2.The method according to claim 1, wherein a thixotropic index of theunhardened first elastic material is larger than that of the unhardenedsecond elastic material.
 3. The method according to claim 2, whereinthixotropic indexes of the unhardened first elastic material and theunhardened second elastic material are respectively larger than about1.7.
 4. The method according to claim 1, wherein thixotropic indexes ofthe unhardened first elastic material and the unhardened second elasticmaterial are respectively larger than about 1.7.
 5. The method accordingto claim 1, wherein at least one of the first elastic material and thesecond elastic material is silicone rubber.
 6. The method according toclaim 1, wherein the step of forming the first elastic material isperformed by one of dropping the first elastic material by one of aniron and a dispensers and dipping the edge portion of the piezoelectricelement into the first elastic material in a fluid state.
 7. The methodaccording to claim 1, wherein the step of forming the second elasticmaterial is performed by dipping the piezoelectric element into thesecond elastic material in a fluid state.
 8. The method according toclaim 1, wherein the step of forming the outer-cladding resin isperformed by dipping the piezoelectric element into the outer-claddingresin in a fluid state.